1. Field of the Invention
The present invention relates in general to a cutting tool assembly, and more particularly to such a cutting tool assembly having a weight which is reduced without reducing rigidity of the tool assembly, for permitting a cutting tool of a large cutting diameter to be used in a small-sized machine tool with a spindle nose having a size of No. 30.
2. Discussion of the Related Art
It is common that a cutting tool such as a face milling cutter is attached to a spindle of a machine tool via a cutting tool holder or arbor. It is also common that the cutting tool is fastened to the arbor by means of a single screw bolt which is adapted to pass through a center hole of the cutting tool and is tightly screwed into an internally threaded hole of the arbor, as defined in JIS and ISO, where the cutting tool has a cutting diameter of not larger than 160 mm. JP-A-H10-6120 (publication of unexamined Japanese Patent Application laid open in 1998) discloses an example of such a cutting tool assembly, particularly, in its paragraph and FIG. 2.
FIG. 3 is a view showing a conventional cutting tool assembly including an arbor 100 and a face milling cutter 200. This cutting tool assembly is designed to be used in a machining center having a spindle nose of BT No. 30 (7/24 taper No. 30). The arbor 100 has a tapered shank portion 101 located in its axially proximal end portion, and an annular flange portion 102 located on the lower side of the tapered shank portion 101. The annular flange portion 102 has a circumferentially extending V-shaped groove which is to be brought into engagement with an arm of an automatic tool changer (ATC) of the machining center.
The arbor 100 further has an axial extension portion 103 located on the lower side of the annular flange portion 102 and axially extending over a predetermined distance, and a positioning boss 104 formed on the center of a lower end face of the axial extension portion 103. A pair of keys 105 are fixed onto an outer circumferential surface of the axial extension portion 103, by suitable fastening means such as screw bolts. The face milling cutter 200 has, in its upper end face, a positioning hole 201 and a pair of slots 202 for receiving the positioning boss 104 and the keys 105.
For fastening the face milling cuter 200 to the arbor 100, the positioning boss 104 and the keys 105 of the arbor 100 are first introduced into the positioning hole 201 and the slots 202 of the cutter 200. A hexagonal-head bolt 300 is then screwed into an internally threaded hole formed in an end face of the positioning boss 104 of the arbor 100, so that the cutter 200 is tightened between the arbor 100 and the bolt 300. While the cutter 200 and the arbor 100 are being thus fastened to each other, the cutter 200 and the arbor 100 are prevented from being rotated relative to each other, owing to the engagements of the slots 202 and the keys 105.
In the manufacture industry these days, there is a demand for reduced sizes of machining equipment, for satisfying requirements for reduction in required floor spaces, energies and costs. In this respect, there is a tendency of replacement of machine tools having spindle noses of BT No.40 with machine tools having spindle noses of BT No.30. However, in general, a machine tool with a spindle nose of BT No.30 is not capable of holding a heavy tool assembly whose weight is larger than about 3 kg, due to the relatively small mechanical rigidity of its components such as its spindle. Due to the limitation in the weight of the tool assembly, a large-sized cutting tool can not be used for the machine tool with the BT No.30 spindle nose. For example, a face milling cutter having a cutting diameter of larger than 80 mm can not be mounted on the machine tool with the BT No.30 spindle nose. That is, a milling operation with a large-sized face milling cutter can not be practiced in the machine tool with the BT No.30 spindle nose.
There are many workpieces each of which can be machined at its surface with a single tool path of a large-sized face milling cutter (e.g., having a cutting diameter of about 125 mm) but can not be machined at its surface with a single tool path of a small-sized face milling cutter (e.g., having a cutting diameter of about 63 mm) which is commonly used in the machine tool with the BT No. 30 spindle nose. That is, in the machining operation with the machine tool of the BT No. 30 spindle nose, such a workpiece has to be machined with successive two or more of tool paths of the face milling cutter, so that a step is inevitably formed along an overlapped portion of the surface which portion has been subjected to a tool path of the cutter and also to the subsequent tool path of the cutter. There are many cases where the face milling is effected in the machine tool of the BT No. 40 spindle nose (for avoiding the formation of the undesirable step on the machined surface) while the other machining (e.g., drilling and tapping) following the face milling is effected in the machine tool of the BT No. 30 spindle nose. Such a transfer of the workpiece between the two machine tools requires additional steps and accordingly leads to deterioration in the machining efficiency.